Echo sounding systems, which are widely used for underwater imaging, work by transmitting a sound pulse or “ping” at a specific frequency, and then receiving that same pulse through a receiver. The received signal is input into a computer processor that determines how long it takes to receive the returning pulse. This delay is translated into depth. The processing system assigns a color range or grey scale for depth to create a 2D or 3D top-side model of the seafloor. U.S. Pat. No. 5,485,432 of Aechter et al., incorporated herein by reference, describes the components and procedures involved in acoustic imaging of the seafloor using a multibeam echosounder.
A typical echo sounding system for mapping the seafloor or the floor of any body of water includes transducers for transmitting and receiving acoustic energy and a set of instruments that measure the location and the orientation of the transducers. Typically, the transducers are mounted on a ship or boat, but they may also be attached to a tow fish or autonomous underwater vehicle. In order to acquire accurate soundings from the seafloor, the instruments that measure the orientation, i.e., the three orthogonal components pitch, roll, and heading, must be corrected for static biases that are introduced by vehicle movement, misalignment and asymmetry in the positioning of the transducers.
The common approach to minimize static biases and calibrate echo sounding systems is known as the “patch test” (Godin, 1996, 1998). During a patch test, the ship (or platform) acquires sets of reciprocal or offset lines over a slope (“bounded slope”) or an object. Cross-sections (profiles) of the surveyed area are represented as curves on depth-distance plots. The plotted curves corresponding to the reciprocal survey lines are overlaid to provide a visual indication of a bias in the orientation, where offsets between the two curves are indicative of a static bias. There are four variables that are typically measured for calibration: time delay, pitch, roll and heading, listed in the order in which they are usually measured. Both time delay and pitch measurement include a 10-20° slope in the seafloor with a flat surface on each side. If offsets exist, the soundings of the slope will shift for reciprocal directions (pitch) or with two lines acquired in the same direction, with the same center line, but at different speeds (time delay). Roll measurement is conducted on a flat bottom in order to indicate the offset between the port and starboard outer beams on reciprocal lines acquired with the same center line but in opposite directions. Roll offset will be shown as a change in sounding height on the same side of the swath for each direction. Heading requires a discrete object or slope in the outer beams of two separate survey lines. If an offset is present, it will be appear as a shift in the object's position. Commercially-available software tools such as Seafloor Information System (SIS) from Kongsberg Maritime (Norway) and HIPS/SIPS from CARIS (New Brunswick, Canada) provide user interfaces that assist in visualization and processing of acoustic data and may be used to calculate offsets. The user enters a correction and observes whether the offset improves or degrades the alignment of the curves, and thus iterates toward a solution.
An alternative technique described in International Patent Publication WO2003/065073 of Bjorke, incorporated herein by reference, uses a quantitative error minimization of the seafloor in common between two surveyed lines.
Both the patch test and the quantitative error minimization approach of Bjorke are limited by the noise inherent to any acoustic surveying system. This noise will limit the ability to achieve a calibration because the user reaches a point at which the size of the correction is less than the scale of the scatter of the soundings. A wider corridor, which increases the number of soundings, also increases the number of noisy soundings. A quantitative correction will also be limited by soundings.